Lyme disease, caused by the arthropod-borne spirochete Borrelia burgdorferi, is a multisystem disorder that can lead to dermatologic, cardiac, neurologic, and rheumatologic manifestations. B. burgdorferi is maintained in a complex enzootic cycle involving its arthropod vector (Ixodes ticks) and a rodent mammalian host. Little is known about how B. burgdorferi adapts to two such distinctly different host environments and causes disease. Our long-term objective is to elucidate molecular mechanisms underlying host adaptation of B. burgdorferi with the expectation that this work will lay the groundwork for the development of innovative approaches for the treatment of and prevention of Lyme disease. In this regard, we have identified a key regulator Rrp2 that is essential for differential gene expression during the spirochete's enzootic cycle. However, the upstream signaling pathway that activates Rrp2 remains unknown. In this proposal, we will employ the temperature-induced Rrp2 activation model to dissect the signaling pathway that leads to the activation of the Rrp2 pathway. In the first aim, we will determine the contributions of acetyl phosphate and the putative histidine kinases to the Rrp2 activation. In the second aim, we will investigate the molecular mechanism underlying constitutive activation of the Rrp2 pathway by the ospAB mutant. PUBLIC HEALTH RELEVANCE: Outcomes of this proposal will elucidate the complex signal sensing mechanisms of the Rrp2 pathway, which will fill a major gap in our understanding of Bb host adaptation. These findings could lead to the developments of therapeutic targets and form a basis for developing strategies to block the enzootic cycle of Bb.